A man covered with variolous pustules, has all over his body a stratum of the very same matter, an atom of which gave him the small-pox a few days before, and the smallest particle of which will inoculate another, if applied to his skin, or, if conveyed with the air into his lungs, may give him a mortal small-pox. Yet this man, so thoroughly coated with the venom, finds no alteration in his health when the suppuration is over, but what proceeds from his past illness; and the matter he is still covered with has no farther power over him.
Suppose a body of such a nature as to be set on fire by a single spark; if, after having seen it in a blaze, you should observe it surrounded with flames, yet neither burnt nor so much as heated by them, would you not fay that it is become incombustible? In like manner, when you have seen the smallest variolous atom by its bare application, infecting a human body, and afterwards behold the same body covered with the same kind of matter, and not in the least affected by it, will you not conclude that it is no longer susceptible of infection, and, if I may so fay, that it is become invariolable?
This property of the variolous matter so active the first time it is applied to a human body and so inert as to the same body, when it has produced its effect, and been propagated and multiplied, ought always to be kept in view, if we would understand any thing of the hitherto unknown nature of this strange disorder.